Electron beam (EB) physical vapour deposited (PVD) thermal barrier coatings(TBCs) have been used in gas turbine engines for a number of years. The primarymode of failure is attributed to oxidation of the bond coat and growth of thethermally grown oxide (TGO), the alumina scale that forms on the bond coat andto which the ceramic top coat adheres. Once the TGO reaches a criticalthickness, the TBC tends to spall and expose the underlying substrate to the hotgases. Erosion is commonly accepted as a secondary failure mechanism, whichthins the TBC thus reducing its insulation capability and increasing the TGOgrowth rate. In severe conditions, erosion can completely remove the TBC overtime, again resulting in the exposure of the substrate, typically Ni-basedsuperalloys. Since engine efficiency is related to turbine entry temperature(TET), there is a constant driving force to increase this temperature. With thisdrive for higher TETs comes corrosion problems for the yttria stabilisedzirconia (YSZ) ceramic topcoat. YSZ is susceptible to attack from moltencalciumâ  magnesiumâ  aluminaâ  silicates (CMAS) which degrades the YSZ bothchemically and micro-structurally. CMAS has a melting point of around 1240 à °Cand since it is common in atmospheric dust it is easily deposited onto gasturbine blades. If the CMAS then melts and penetrates into the ceramic, the lifeof the TBC can be significantly reduced. This paper discusses the variousfailure mechanisms associated with the erosion, corrosion anderosionâ  corrosion of EB PVD TBCs. The concept of a dimensionless ratio D/d,where D is the contact footprint diameter and d is the column diameter, as ameans of determining the erosion mechanism is introduced and discussed for EB
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